The present invention relates to an optical semiconductor device, and more particularly to an optical semiconductor device suitable for use in an optical communication module, optical communication system or optical network.
One crystal growth step suffices for the fabrication of a ridge-waveguide device. Therefore, the fabrication process of the ridge-waveguide device is very simple as compared with that of a buried-hetero-structure device. Hitherto, a satisfactory device reliability has been reported about an indium phosphide-based ridge waveguide lasers. However, the conventional indium phosphide ridge lasers using a ridge (denoted by reference numeral 70 in FIG. 9) with a rectangular cross section formed by use of a wet etching technique with hydrochloric acid involves the following problems.
(1) Since the width of an electrode contact on an active layer and the width of a light emitting layer of the active layer are substantially the same, it is required from the aspect of reduction in resistance of the device that the width of the cross section of the ridge providing the width of a light emitting region should be set to a value equal to or larger than 2 .mu.m. Therefore, it is difficult to realize the stabilization of a transverse mode and the reduction of the threshold current to a value not larger than 10 mA. Further, since the device resistance is relatively large, a high-output operation is limited due to a thermal saturation phenomenon.
(2) Since it is difficult to make the width of the light emitting region narrow, it is difficult to reduce a parasitic capacitance of the device.
(3) A lithographic alignment precision required in providing an insulating layer window for an electrode contact on the ridge is very severe.
Techniques relevant to the ridge waveguide lasers have been disclosed by Institute of Electronics, Information and Communication Engineers of Japan, '93 Spring Conference C-159, March 1993.